This is because HVAC companies wildly overcharge. It does not take $1,000 worth of labor to drain and refill refrigerant, but it does take a qualified technician, and there are not enough of those.
That’s absolutely true. But the same problem would exist, much worse, if everyone started modifying their old A/C systems to turn them into heat pumps.
To add to the list of why this whole idea seems dubious: a conventional air conditioner operates with its evaporator coil at a balmy 50F or so. If you reverse it, the outdoor coil will become the evaporator, and it will operate below freezing if it’s cold out (even if the outdoor temperature is a bit above freezing). As a result, condensate can form and freeze on the coil.
This is entirely manageable (any modern heat pump can operate in decently chilly conditions, and heat pumps equipped for low temperature operation can operate in very cold conditions), but I expect that at least some engineering is involved.
